The present invention relates to the phenomenon known as a fiber fuse, and in particular a method of controlling a network to reduce the likelihood of a fiber fuse being initiated.
Optical power levels in optical transmission systems are generally increasing. This is due to a number of factors.
For instance, optical transmission systems, including optical fibers and other optical devices such as polarisation mode dispersion compensation mechanisms and photonic switches, all have attenuation characteristics. Increasing the power or an optical signal provides a better signal to background noise ratio, and allows the signal to be transmitted longer distances over the optical transmission system before optical amplification is required. Advances in laser technology have ensured that higher powered lasers are now more readily and cheaply available, thus allowing a cost effective implementation of high optical power signal generation.
Typical optical transmission systems simultaneously transmit data using a multitude of different wavelengths, each transmission channel having a separate wavelength of light for transmission of the respective optical signal. Increasingly, channels are becoming more closely packed together with regard to wavelength e.g. DWDM (Dense Wavelength Division Multiplexed) systems. Increasing the number of simultaneous optical transmissions at different wavelengths will increase the average optical power being carried by the transmission system.
Many optical systems utilise optical amplifiers comprising optical fiber. An example of this is a Raman amplifier i.e. an amplifier that utilises the Raman effect. Optical amplifiers of this type normally use relatively high power pump laser, for providing the optical power that is utilised to amplify the optical signal power. Current trends indicate it is increasingly likely that Raman amplifiers will be utilised in future telecommunications systems.
Experiments have indicated that high optical powers propagating through fibers can induce an effect referred to as a xe2x80x9cfiber fusexe2x80x9d. The fiber fuse effect, also termed self-propelled self-focusing (SPSF), is a catastrophic damage mechanism. Electronics letters, Jan. 7, 1988. Vol. No. 1, pages 47-48 by R Kashyap and K J Blow and Electronics letters Jan. 5, 1989, Vol. 25, No.1, Pages 33-34 by D P Hand and T A Birks describe this phenomena in some detail and describe a fiber fuse damage circuit-breaker, and are incorporated herein by reference.
The fiber fuse effect is believed to be initiated by local heating of the fiber. This can lead to a runway thermal effect which, provided the laser power is sufficient, continues until the fiber core melts. A thermal shock wave is created (visible as a bright spot of side-scattered light) that propagates back along the fiber towards the optical power source. This results in the fiber being permanently damaged and unable to guide light.
Propagation velocity is believed to be of the order of tens of meters per second. A fiber fuse occurring in a telecommunications system could be extremely damaging. Additionally, in systems where optical fiber spans (i.e. typically the length between optical fiber amplifiers) are of the order of 80 kilometers, it will be appreciated that if the fiber fuse is not contained it has the capacity to damage large lengths of optical fiber. This would require replacement of the damaged fiber. If the fiber fuse is able to propagate into optical processing equipment, such as an amplifier or pump laser, the fiber fuse can result in damage to very expensive network components.
It is therefore desirable to limit the damage caused by fiber fuses or to reduce significantly the risk of a fiber fuse developing. As mentioned above, it has been proposed that the initiation of a fiber fuse results from local heating of the fiber. How this local heating is initiated has not been fully understood, although it has been recognised that a fiber fuse may be initiated at the site of fiber damage, such as a fiber break.
The fiber fuse effect is also discussed in the assignee""s copending U.S. patent application Ser. No. 09/544,362, filed Apr. 6, 2000 entitled xe2x80x9cFiber Fuse Protectionxe2x80x9d which is incorporated herein by way of reference material,
There is a need to predict the conditions under which a fiber fuse is likely to be initiated in order to enable corrective action to be taken.
The invention is based on the realisation that a fiber fuse can only be initiated within a fiber when the fiber is carrying power greater than a threshold power. This threshold power is a function of the fiber type.
In one aspect, the present invention provides a method of routing of signals through an optical network, comprising determining a route through the network for which the power level within each branch of the network along the route is below a threshold power level which is a function of the fiber characteristics of the branch.
This method enables power levels within the network to be controlled such that a threshold power is not exceeded, which threshold power is considered to be the power below which a fiber fuse can not be initiated. The realisation that this threshold power level exists is the result of prolonged study of operating conditions leading to a fiber fuse.
In particular, it has been found that the threshold power is a function of the core diameter and the higher mode cutoff wavelength of a fiber. For large core diameters, an increase in core diameter increases the threshold power. This is considered to result from reduced xe2x80x9cthermal lensingxe2x80x9d. This is one phenomenon which can be used to explain the propagation of the thermal fuse, and is based on the idea that a local fuse location is imaged to a focal point within the core at an adjacent location, at which a fuse is created. Increasing the core diameter, and therefore the mode field diameter, increases the size of these focal points and thereby reduces the intensity. For small core diameters, it is believed that heat dissipates more readily to the cladding, so that as the core diameter is reduced, the threshold also increases. Thus, the threshold power has a minima value with respect to core diameter (for a fiber of constant cutoff wavelength)
In a another aspect, the invention provides a method of controlling the routing of signals through an optical network, comprising:
determining a route through the network;
estimating the power level within each branch of the network along the route and determining the fiber characteristics of each branch of the network along the route;
comparing the power level for each branch with a threshold power level, wherein the threshold power level is a function of the fiber characteristics of the branch; and
if the power exceeds the threshold power level, determining an alternative route through the network.
This routing method ensures that the route does not result in the threshold power being exceeded in any branch of the network. This method may only be possible if the network has been desired to support a volume of optical traffic flow which can be routed in this manner. If the invention is to be applied to existing networks, it may not be possible to ensure that the threshold power levels are not exceeded at any location in the network.
Therefore, in a further aspect, the invention provides a method of monitoring an optical network to predict the initiation of a fiber fuse, comprising:
measuring the power level within each branch of the network and determining the fiber characteristics of each branch of the network; and
comparing the power level for each branch with a threshold power level, wherein the threshold power level is a function of the fiber characteristics of the branch.
In this method, locations within a network where a fiber fuse is most likely to be initiated are identified. This then enables specific monitoring to be applied to that location with the network. For example, local losses at the location may be measured. For example, if an increase in local loss is detected, power levels of the signals provided to the network can be reduced. An increase in local losses may indicate degradation of components, which can lead to a fiber fine being initiated.
In a further aspects, the invention also provides a computer program product storing a computer program for implementing a method of monitoring all optical network to predict the initiation of a fiber fuse, the method comprising:
measuring the power level with each branch of the network and determining the fiber characteristics of each brat of the network; and
comparing the power level for each branch with a threshold power level, wherein the threshold power level is a function of the fiber characteristics of the branch.
According to a still further aspect of the invention, there is provided an of network management system comprising:
power monitoring equipment for monitoring the power it branches of the network;
a threshold generator for generating threshold powers below which a fiber fuse is not expected to be initiated for different types of fiber within the network, wherein the threshold powers are a function of the fiber characteristics; and
a comparator for comparing the threshold powers with the monitored powers.
This management system enables the routing to be controlled to premium power levels exceeding the threshold, or enables other measures to be taken. The threshold power level may again be a function of the core diameter and the higher mode cutoff wavelength of the fiber of the branch, and again represents minimum power at which a fiber fuse can be initiated.